Operation of lean burn engines, e.g., diesel engines and lean burn gasoline engines, provide the user with excellent fuel economy due to their operation at high air/fuel ratios under fuel lean conditions. Diesel engines, in particular, also offer significant advantages over gasoline engines in terms of their fuel economy, durability, and their ability to generate high torque at low speed.
From the standpoint of emissions, however, diesel engines present problems more severe than their spark-ignition counterparts. Emission problems relate to particulate matter (PM), nitrogen oxides (NOx), unburned hydrocarbons (HC) and carbon monoxide (CO). NOx is a term used to describe various chemical species of nitrogen oxides, including nitrogen monoxide (NO) and nitrogen dioxide (NO2), among others.
Oxidation catalysts comprising a precious metal dispersed on a refractory metal oxide support are known for use in treating the exhaust of diesel engines in order to catalyze the oxidation of both hydrocarbon and carbon monoxide gaseous pollutants to carbon dioxide and water. Such catalysts have been generally contained in units called diesel oxidation catalysts (DOC), or more simply catalytic converters, which are placed in the exhaust flow path from a Diesel-powered engine to treat the exhaust before it vents to the atmosphere. Typically, the diesel oxidation catalysts are formed on ceramic or metallic substrate carriers (such as the flow-through monolith carrier) upon which one or more catalyst coating compositions are deposited. In addition to the conversions of gaseous HC, CO and the SOF (Soluble Organic Fraction) of particulate matter, oxidation catalysts that contain platinum group metals (which are typically dispersed on a refractory oxide support) promote the oxidation of nitric oxide (NO) to NO2.
Catalysts used to treat the exhaust of internal combustion engines are less effective during periods of relatively low temperature operation, such as the initial cold-start period of engine operation, because the engine exhaust is not at a temperature sufficiently high for efficient catalytic conversion of noxious components in the exhaust. To this end, an adsorbent material, which may be a molecular sieve, for example, a zeolite, may be provided as part of a catalytic treatment system in order to adsorb gaseous pollutants, usually hydrocarbons, and retain them during the initial cold-start period. As the exhaust gas temperature increases, the adsorbed hydrocarbons are driven from the adsorbent and subjected to catalytic treatment at the higher temperature.
One effective method to reduce NOx from the exhaust of lean-burn engines, such as gasoline direct injection and partial lean-burn engines, as well as from diesel engines, requires trapping and storing of NOx under lean burn engine operating conditions and reducing the trapped NOx under stoichiometric or rich engine operating conditions or lean engine operating with external fuel injected in the exhaust to induce rich conditions. The lean operating cycle is typically between 1 minute and 20 minutes and the rich operating cycle is typically short (1 to 10 seconds) to preserve as much fuel as possible. To enhance NOx conversion efficiency, short and frequent regeneration is favored over long but less frequent regeneration. Thus, a lean NOx trap catalyst generally must provide a NOx trapping function and a three-way conversion function.
Some lean NOx trap (LNT) systems contain alkaline earth elements. For example, NOx sorbent components include alkaline earth metal oxides, such as oxides of Mg, Ca, Sr and Ba. Other lean LNT systems can contain rare earth metal oxides such as oxides of Ce, La, Pr and Nd. The NOx sorbents can be used in combination with precious metal catalysts such as platinum dispersed on an alumina support in the purification of exhaust gas from an internal combustion engine.
A conventional LNT typically contains basic sorbent components (e.g., BaO/BaCO3 and/or CeO2) for NOx storage and platinum group metals (PGM, i.e., Pt, Pd and Rh) for catalytic NOx oxidation and reduction. The LNT catalyst operates under cyclic lean (trapping mode) and rich (regeneration mode) exhaust conditions during which the engine out NO is converted to N2 as shown in equations 1-6:Lean condition: 2NO+O2→2NO2  (1)(Trapping mode) 4NO2+2MCO3+O2→2M(NO3)2+2CO2  (2)Rich condition: M(NO3)2+2CO→MCO3+NO2+NO+CO2  (3)(Regeneration mode) NO2+CO→NO+CO2  (4)2NO+2CO→N2+2CO2  (5)2NO+2H2→N2+2H2O  (6)
Molecular sieves such as zeolites are used in diesel oxidation catalyst (DOC) and, as noted above, in Lean NOx Trap (LNT) applications for the purpose of adsorbing hydrocarbons (HC) from the engine exhaust during startup of the vehicle when the catalyst is cold and unable to oxidize the hydrocarbons to CO2 (cold start). When the temperature of the exhaust increases to the point when the precious metal in the catalyst becomes active, hydrocarbon is released from the molecular sieve and is subsequently oxidized to CO2. There are numerous strategies and methods for combining zeolite and precious metal in a DOC or LNT catalyst formulation. For instance, molecular sieve can be combined in the same layer as the precious metal or separated into different layers. For DOC applications, Pt and Pd are frequently used platinum group metals for oxidation of carbon monoxide (CO) and hydrocarbons (HC) present in diesel engine exhaust. The choice of these active metals is due to a combination of performance (i.e. mixtures of Pt and Pd have improved performance when compared to Pt and Pd alone) and cost (i.e. the price of Pd is significantly less than that of Pt). A frequently used zeolite in diesel applications for HC adsorption is Beta zeolite due to its high capacity for storage of hydrocarbons typically found in diesel exhaust. However, when beta zeolite available from most commercial suppliers is combined with Pt/Pd DOC catalyst in the same slurry and coating layer, CO oxidation performance of the catalyst is reduced significantly compared to that of Pt/Pd DOC catalyst without zeolite addition. Although HC performance is improved due to the HC storage function of the zeolite, the CO performance is reduced due to a negative interaction between the Pt/Pd/alumina catalyst and zeolite. One method to avoid this negative interaction is to separate the Pt/Pd and zeolite into different coating layers. However, it is desirable for simplicity of slurry preparation and monolith coating to combine the Pt/Pd and zeolite in a single slurry and/or coating layer. In order to accomplish this, a new method for overcoming the negative interaction between Pt/Pd and zeolite on CO oxidation performance must be found.